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Questions and Answers
What is the difference between calcium sulfate hemihydrate and calcium sulfate dihydrate?
What is the difference between calcium sulfate hemihydrate and calcium sulfate dihydrate?
Which type of hemihydrate is used for plaster?
Which type of hemihydrate is used for plaster?
What is the process of heating gypsum to drive off water called?
What is the process of heating gypsum to drive off water called?
The amount of water needed for the chemical reaction of setting gypsum is the same for plaster, stone, and die stone.
The amount of water needed for the chemical reaction of setting gypsum is the same for plaster, stone, and die stone.
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What is the name given to the points where "arms" grow in the gypsum setting process?
What is the name given to the points where "arms" grow in the gypsum setting process?
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What is the effect of using less water in a gypsum mix?
What is the effect of using less water in a gypsum mix?
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What is the main advantage of polysulfide impression material?
What is the main advantage of polysulfide impression material?
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Polyether impression material is hydrophobic.
Polyether impression material is hydrophobic.
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Which type of impression material is known for its ability to shrink after setting?
Which type of impression material is known for its ability to shrink after setting?
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What is the term used for the separation of liquid from a colloid causing shrinkage?
What is the term used for the separation of liquid from a colloid causing shrinkage?
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What is the term used for the phase that is distributed throughout the dispersion medium?
What is the term used for the phase that is distributed throughout the dispersion medium?
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Study Notes
Dental Materials - Calcium Sulfate
- Calcium sulfate hemihydrate (CaSO4·1½H2O) is a powder form.
- Calcium sulfate dihydrate (CaSO4·2H2O) is hardened stone. The difference is the water molecules. Anhydrous/anhydrate is heated stone with no water, making it weak and crumbly.
- Plaster is beta-hemihydrate.
- Stone and die stone is alpha-hemihydrate.
- Die stone is boiled in a 30% calcium solution.
- Water released as steam is from water crystallization.
Calcining
- Calcining is the process of heating gypsum to drive off water.
- Plaster forms large, irregular, porous crystals.
- Stone forms smaller, denser, regular sized crystals.
Mixing Ratios
- Plaster: 50-55 ml water
- Stone: 32-40 ml water
- Die stone: 22-24 ml water
- The amount of water needed for chemical reaction is the same at 18.6 ml
- W/P ratio is 0.5, 0.3, 0.24
Setting Reaction
- CaSO4·½H2O + 1½H2O → CaSO4·2H2O + heat
- Heat released equals the heat generated during calcining at 25 degrees.
- Expansion through seeds of crystallization called "nuclei"; they grow arms like snowflakes.
Controlling Setting Time
- Controlling setting time factors include temperature, W/P ratio, and amount of mixing.
Chemical Additives
- Lower water content creates a thicker mix, accelerated set and increased expansion. The result is a harder final set.
Polysulfide
- Excellent for areas with undercuts.
- Used for dentures.
- Working time: 5 minutes
- Setting time: 10 minutes plus.
- Smells bad
- High tear resistance.
Polyether
- Two-paste system.
- Low molecular weight polyether + catalyst.
- Suitable for areas where moisture control is difficult.
- It swells in high humidity.
- Used for crowns, bridges, partial dentures, and implants.
Silicone
- Rigid when set (bad for undercuts), absorbs water.
- Upgrade from polyether.
- Made by addition reactions.
- Equal parts, no byproduct.
Impression Materials
- Working time 2:30
- Setting time 5 min.
- Adequate setting time for monophase impressions.
- Rigid when set (bad for undercuts), absorbs water
- made as an upgrade from polyether
- Condensation reaction
- Hydrophobic
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Description
Test your knowledge on calcium sulfate and its variants in dental materials. This quiz covers topics such as the properties of gypsum, the calcining process, mixing ratios, and the setting reaction. Perfect for students studying dental material science!
